Radiation-sensitive compositions are routinely used in the preparation of imageable materials including lithographic printing plate precursors. Such compositions generally include a radically polymerizable component, radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties, imaging performance, and image characteristics.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged and/or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the unexposed regions become an image.
Various radiation compositions and imageable elements are described in U.S. Pat. No. 6,569,603 (Furukawa) and EP 1,182,033A1 (Fujimaki et al.). Other IR-sensitive compositions are described in U.S. Pat. No. 6,309,792 (Hauck et al.), U.S. Pat. No. 6,893,797 (Munnelly et al.), U.S. Pat. No. 6,787,281 (Tao et al.), and U.S. Pat. No. 6,899,994 (Huang et al.), U.S. Patent Application Publication 2003/0118939 (West et al.), and EP 1,079,276A1 (Lifka et al.) and EP 1,449,650A1 (Goto). Still other imaging compositions and elements are described in U.S. Pat. No. 6,858,373 (Kunita) and U.S. Patent Application Publication 2004/0043325 (Shibuya et al.).
Negative-working imaging compositions and element can have components that generate free radicals and other components that react, polymerize, or crosslink in the presence of those free radicals. Initiator compositions that will produce free radicals in response to irradiation (such as UV, visible, or infrared irradiation) are known in the art. For example, U.S. Pat. No. 6,936,384 (Munnelly et al.) describes the use of compounds that produce free radicals response to infrared radiation, such as borates, alkyltriarylborates, triazines, and onium salts including iodonium salts. This reference also describes the presence of metallocenes to maintain imaging speed after accelerated aging tests. The imageable elements described in this reference also included a poly(vinyl alcohol) oxygen barrier layer over the imageable layer. It is also known that alkyltriarylborates can provide free radicals for radiation-sensitive imageable compositions but that such compositions have poor shelf-life.
The use of titanocenes are also described for UV and visible radiation-sensitive imaging compositions in, for example, U.S. Pat. No. 6,010,824 (Komano et al.), U.S. Pat. No. 6,818,372 (Kunita et al.), U.S. Pat. No. 7,1,108,952 (Sugasaki et al.), and U.S. Pat. No. 7,122,293 (Sugashi et al.), and U.S. Patent Application Publication 2006/0046189 (Kunita et al.).